class Solution {

public:

    int countPrimes(int n) {

        int num = ;

        if(n < ) return num;

        int i, j, index;

        isPrime = new bool [n];

        isPrime[]=false;

        isPrime[]=false;

        for(i = ; i < n; i++){

            isPrime[i] = true;//initialize as true, means all are primes

        }

        for(i = ; i < n; i++){

            if(!isPrime[i]) continue;

            num++;

            for(j=; i*j < n; j++){

                isPrime[i*j] = false;

            }

        } 

        return num;

    }

private:

   bool* isPrime;

};